[RISCV] Fix crash reported in #109313

Change edc71e2 had tried to handle the case
where an instruction wasn't available, but had used a dyn_cast instead of
a dyn_cast_or_null.  Switch instead to an explicit null check, and a cast.

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1051,13 +1051,12 @@ RISCVTTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
   // vp load/store
   case Intrinsic::vp_load:
   case Intrinsic::vp_store: {
+    if (!ICA.getInst())
+      break;
     Intrinsic::ID IID = ICA.getID();
     std::optional<unsigned> FOp = VPIntrinsic::getFunctionalOpcodeForVP(IID);
-    auto *UI = dyn_cast<VPIntrinsic>(ICA.getInst());
-
-    if (!UI)
-      break;
     assert(FOp.has_value());
+    auto *UI = cast<VPIntrinsic>(ICA.getInst());
     if (ICA.getID() == Intrinsic::vp_load)
       return getMemoryOpCost(
           *FOp, ICA.getReturnType(), UI->getPointerAlignment(),